Dental x-rays are typically taken with a film that is placed in the patient's mouth. The film is exposed through the teeth by an x-ray source that resides outside the patient's head. While this method has been in use for many years, it has its disadvantages. First, the patient is exposed to a significant dose of x-rays. This dose is accumulative over the patient's lifetime. Second, the time, cost, and equipment needed to process the film increases the cost of the dental examination. Third, the chemicals utilized in processing the film pose a disposal problem.
These problems have led to several attempts to replace the film component of the traditional x-ray examination with a solid-state sensor that is placed in the patient's mouth to record the x-ray image. In such systems, a layer of scintillation material is used to convert the x-rays to visible light. The visible light is then imaged onto a solid-state imaging array. Since solid-state x-ray sensors of this type are significantly more sensitive to x-rays than the films utilized today, the x-ray dosage can be reduced, typically, by a factor of 10. In addition, the sensor is re-used, and hence, the cost and disposal problems associated with the conventional x-ray system are avoided. Finally, since the image is in digital form, systems based on solid-state sensors are easily adapted to paperless office systems.
Unfortunately, these sensors are much thicker than the conventional film based sensors and the resolution of the sensors is also less than that of conventional film-based sensors. The sensors include a channel plate between the scintillation material and the image-recording element, which is typically a silicon-based imaging array. When an x-ray is converted in a pixel of the imaging array, the resultant signal can be much greater than the signal produced by the light from the scintillator. The probability of such a conversion event is small, and hence, the x-ray hits result in scattered bright pixels in the image that render the image objectionable. To reduce these events, a layer of shielding material that transmits the light from the scintillator to the imaging array is used. The shielding layer typically consists of a bundle of optical fibers that images the surface of the scintillator onto the surface of the imaging array. The optical fibers are doped with a heavy metal that absorbs x-rays that are not converted in the scintillation material. The shielding layer blocks most of the x-rays from reaching the image sensor, and hence, reduces the number of bright pixels to an acceptable level.
While the shielding layer solves the bright pixel problem, it introduces new problems. The shielding plate is typically greater than 2.5 mm in thickness, and hence, significantly increases the thickness of the apparatus that is placed in the patient's mouth. The increased thickness is objectionable to many patients.